1. Field of the Invention
The present invention relates to carbon dioxide gas sensors used in automated chemical analysis and immunological testing. More particularly, the present invention relates to fiber optic sensors incorporating an improved carbon dioxide sensing element with a dye indicator in a polymer matrix suitable to be used in vivo in the body of a human or other animal.
2. Description of Related Art
The development and use of fiber optic sensors is a fast growing and competitive field. The first references to fiber optic sensors for detecting and quantitatively measuring gas concentrations emerged in the mid 1970's. Recent advances in fiber optic technology have increased the activity in the development and use of such sensors.
There is a strong interest in the medical field in developing in vivo sensors. Early efforts were applied in developing in vitro sensors and sensors for industrial uses. Many design criteria are important to the development of in vivo fiber optic sensor elements, requiring consideration of factors such as those related to the inability to control or prepare the sample, and the need to minimize the impact of the sensor's presence on the host organism. Fiber optic sensors must be biocompatible; the sensor must not adversely affect the host organism and the host organism must not adversely affect the sensor. The sensor must be small enough to be compatible with standard arterial devices, such as catheters. Moreover, the sensor must be engineered to be sterile, non-toxic, non-pyrogenic, and as blood compatible as possible.
Fiber optic sensors also must be economically integrable with conventional fiber optic apparatus. It is desirable that the fiber optic sensors be disposable and, therefore, economically inexpensive. Consequently, such sensors must be easy to manufacture, reproducible and simple to calibrate. Although other fiber optic sensors using polymers and indicating dyes have been proposed, many such sensors are too expensive to be considered disposable. Since the polymer chemistry of the sensor requires tedious and time-consuming manufacturing techniques, this also can make the sensors expensive and complex to manufacture, further limiting their use as a disposable.
Carbon dioxide fiber optic sensors are generally based on pH sensor technology. Such carbon dioxide sensors utilize hydrophilic polymers combined with pH fiber optic gas sensors to immobilize a hydrogen ion sensitive dye which is responsive to the carbon dioxide-bicarbonate equilibrium in an aqueous bicarbonate solution exposed to the dye. Semipermeable membranes are used to filter compromising artifacts and other analytes.
Conventional carbon dioxide sensors have incorporated reflectance, absorbance, and fluorescence dye indicators. Although reflectance indicators are well known, they are seldom used in pH or carbon dioxide sensors. A variety of polymeric materials have also been used to immobilize the carbon dioxide indicators for use in fiber optic sensors. When certain absorption indicators, such as phenol red, are in solution with polymers undergoing polymerization, such as acrylamide, the indicator becomes chemically bonded to the polymeric product. Covalent bonding with a polymeric substrate prevents the dye from being washed out of the polymer when exposed to an aqueous environment, such as blood. Covalently bonding the dye or indicator in a hydrophilic polymer substrate is often difficult to achieve and requires additional chemistries to obtain proper covalent bonds. Attempts to eliminate the chemical bonding requirement include creating micro-compartments within the polymer substrate. Covalently bonding a carbon dioxide dye indicator to a polymer and inclusion of a buffer system in the polymeric matrix have heretofore been a difficult, time consuming procedure. It would therefore be desirable to provide an alternative, effective and easy method of making a carbon dioxide sensor incorporating an indicator dye which will not wash out or leach out of the sensor.
Polymeric materials have also been used as a matrix for simply immobilizing fluorescent indicators. Such polymeric materials containing immobilized fluorescent dyes (for example, siloxane-based precursors and fluorescein derivatives) can be bonded to the distal end of optic fibers to provide a gas sensor. Since the indicators respond to any hydrogen ion artifact, such carbon dioxide sensors usually contain a carbon dioxide permeable, hydrophobic membrane. However, curing such hydrophobic membranes often created problems, since heat treatments to cure the polymeric dye indicator matrix can compromise the integrity of the sensor dye indicator material. Production methods known in the art can also create holes and other imperfections which may adversely affect the sensor. Moreover, the polymerization process and subsequent curing methods can often be expensive, cumbersome and unduly time extensive. Conventional fiber optic sensors incorporating polymers in a sensing matrix have used addition-type or condensation-type polymerization techniques to form the basic matrix material. Condensation and addition-type reactions require multiple monomers, catalysts and/or precursors to create the final form of the polymer. Moreover, such reactions are often time consuming, difficult to execute with precision, and are more expensive due to their multicomponent nature. Consequently, a need exists for a fiber optic CO.sub.2 sensor that is easy to manufacture, has consistent geometry and chemistry, is easy to sterilize, is economically produced so that it can be disposable, and can reliably be used in vivo.